The present invention relates generally to printed circuit boards and printed wiring boards for use in the electronics industry. More specifically, the present invention relates to a printed circuit board having at least one embedded capacitor. In one application, such capacitors may be used as decoupling capacitors for integrated circuits installed on the printed circuit board.
The electronics industry currently makes wide use of printed circuit and printed wiring boards (hereafter, collectively referred to as xe2x80x9cprinted circuit boardsxe2x80x9d or xe2x80x9cPCBsxe2x80x9d). Typical printed circuit boards can include multiple composite layers formed from organic and inorganic materials and include both internal and external wiring. The boards allow electrical components to be mechanically supported and electrically connected among one another. As electronic technology advances, the trend is toward placing increasing numbers of interconnect layers, greater pass-through hole densities (permitted by small hole diameters), and finer line (trace) widths on the boards. Each of these is intended to allow a greater number of devices to be installed on a printed circuit board having a given size.
Despite the development of printed circuit boards which allow greater device densities, little progress has been made regarding the ability to build active or passive electronic devices as integrated elements during manufacture of a multi-layer printed circuit board. This is a result of numerous problems that are associated with device and board integration. For example, printed circuit board manufacturing processes and circuit manufacturing processes are substantially incompatible with regard to the required degree of cleanliness, thermal cycling, photolithography and other requirements.
In applications in which a printed circuit board is intended to carry a large number of integrated circuit devices, a correspondingly large number of decoupling capacitors is required. The decoupling capacitors provide instantaneous current requirements for the integrated circuits and also serve to reduce system noise. Unfortunately, the numerous decoupling capacitors can occupy considerable printed circuit board surface area, and at the same time require extra assembly of the overall device in that they must be positioned on and affixed to the printed circuit board. As a result, the requirement for decoupling capacitors adds significantly to the overall cost of manufacture of electronic devices.
Recently, methods for burying a capacitor within the circuit board have been developed. Of these methods, two are currently acceptable from a practical commercial standpoint. The first method uses a thin, substantially copper clad, epoxy-impregnated fiberglass laminate as a parallel plate capacitor. Although the overall dielectric constant of the composite material, (i.e. the fiberglass laminate), is relatively low, (approximately 4.5), the capacitance is still high enough to be effective in some cases. This is particularly true when the laminate used is relatively thin, (i.e., on the order of approximately 0.002 inches). The second method for burying a capacitor within a circuit board involves the application of a filled epoxy containing a high percentage of ceramic filler or pre-fired ceramic forming materials to a roll of copper foil. (As used herein, the term xe2x80x9cceramic fillerxe2x80x9d is intended to encompass pre-fired ceramic forming materials as well as ceramic fillers). The roll can then be cut into sheets, positioned in a face-to-face relationship with the filled epoxy surfaces contacting each other, and then exposed to elevated temperatures and pressures to form a laminate. The overall capacitance of this type of buried capacitor is approximately four times that of the epoxy-fiberglass parallel plate type described above. This is a result of the presence of the ceramic which imparts a high dielectric constant to the laminate body. Unfortunately, however, this method produces a laminate that is approximately 4-5 mils thick because each of the sheets used is approximately 2.5 mils thick and two such sheets must be pressed together in order to achieve the proper bond strength. Likewise, the overall strength requirements of the laminate lead to such thicknesses in order to produce a structure that is sufficiently strong to be processed using standard printed circuit board fabrication methods.
Thus, it should be apparent that a need exists for a simple, low cost method of providing a low-profile integrated (i.e., buried) capacitor in a printed circuit board, and a need also exists for integrated capacitors which have a capacitance that exceeds the capabilities of buried capacitors currently known in the art.
The present invention relates generally to printed circuit boards having one or more integrated or buried capacitors. More particularly, the present invention relates to a capacitor foil for use in forming buried parallel plate capacitors on a printed circuit board intermediate. The capacitor foil includes a conductive layer, a relatively high dielectric constant layer, and a bonding layer. The bonding layer may or may not have a dielectric constant that is lower than that of the layer applied to the copper. The conductive layer may be used to define the power plane of a capacitor and the dielectric and bonding layers may be used to define the dielectric layers of a capacitor. Each of the dielectric and bonding layers are formed of an epoxy or other polymer resin system typically used in the printed circuit board industry, (i.e., polyimides, Bismaleimide triazines, cyanate esters, etc.), however, the dielectric layer is filled with ceramic particles or pre-fired ceramic forming particles, whereas the polymer resin comprising the bonding layer can be unfilled, less highly filled, or as highly filled but relatively uncured. It is noted that the term xe2x80x9cdielectric layerxe2x80x9d as used throughout the specification herein is intended to describe a layer of material having a relatively high dielectric constant.
In use, the capacitor film may be applied to the surface of a laminate which contains numerous copper patterns, each defining, for example, a ground plane of a discrete capacitor, and becoming an inner layer once the capacitor film is in place. By laminating the capacitor foil over the patterned inner layer, a PCB intermediate having a multiplicity of buried capacitors can be formed. Alternatively, if the patterned inner layer is such that there is one solid, contiguous sheet of copper, one large capacitor is formed. If such a single, large capacitor is formed, the capacitance can be shared among all parts of the board. The intermediate may subsequently be processed using any of a wide variety of PCB processing steps in order to fabricate an electronic device based upon a PCB having internal capacitance.